As a product in which a metal wire is electroplated, there is a steel cord for tires. In producing this steel cord, a steel wire is generally subjected to copper electroplating and zinc electroplating. In these electroplating processes, a plurality of metal wires are run along electrode plates placed in an electroplating tank, the surface of each metal wire is electroplated through passing in an electroplating solution of the tank. The electrode plates conventionally used for such wire electroplating are soluble electrodes.
In an electroplating using a soluble anode, as the soluble anode, a metal plate of the same material as the electroplated metal is used, the metal plate itself dissolves in an electroplating solution by anodic dissolution when applying current to supply electroplating metal ions. In this method, there is a problem on quality control that dissolution of electrode plate varies a distance between the plate and metal wire as a cathode, and leads to changes in electroplating thickness with time, so that it is difficult to obtain stable quality. There is also a problem on working efficiency that electrodes must be frequently replaced. In view of these situations, recently, an insoluble anode has been increasingly used in place of a soluble anode.
In a method of electroplating a metal wire using an insoluble anode, since supply of electroplating metal ions from electrode plates cannot be expected, it is necessary to equip a means for supplying the electroplating metal ions additionally. FIG. 3 shows an outline of an electroplating apparatus generally used in an electroplating method using insoluble anodes. In the apparatus shown in FIG. 3, an insoluble electrode plate 3 is horizontally placed at the bottom of an electroplating tank 2 holding an electroplating solution 1. The electroplating solution 1 is overflowed from the electroplating tank 2, a metal wire 5 is passed in the electroplating tank 2 while it is held below the liquid level of the electroplating solution 1 by guide rolls 4 placed back and forth across the electroplating tank 2. In this way, a voltage is applied between the metal wire 5 and the electrode plate 3 by a power supplying means 6. The electroplating solution 1 overflowed from the electroplating tank 2 is collected in an auxiliary tank 7, fed back to the electroplating tank 2 by a pump. An electroplating metal in the electroplating solution being consumed in accompanying with the development of electroplating operation is suitably replenished by a supplying means not shown in the figure.
In such an electroplating apparatus, an electrode plate faces a metal wire passing through in an electroplating solution only from the under side. Since the upper side of a metal wire is open, there are merits that an electrode plate does not disturb a wire-passing operation as well as the apparatus is simple, further, releasing property of gas generated with an electroplating reaction in the electroplating tank is also good. However, there is a problem on quality of electroplating that an electroplating amount on the upper surface is small compared with the under surface facing the electrode plate, the distribution of electroplating amounts in a circumferential direction of wire tends to be uneven.
As a method to solve the problems while keeping the merits of the foregoing electroplating apparatus, there is an electroplating method described in Patent document 1 that two electrode plates are placed opposite so as to sandwich a wire travel path in an electroplating tank from both sides and a metal wire is passed between the electrode plates in both sides. According to this method, as well as the uniformity in distribution of electroplating amount in a circumferential direction of wire is improved, the foregoing merits are taken over as they are since the upper side of wire travel path is opened. In the case where a plurality of metal wires are simultaneously electroplated, the same document describes a mode that a metal wire is passed through each gap between a plurality of electrode plates placed at predetermined intervals.    Patent document 1: Japanese Unexamined Patent Publication No. 2000-192291
In order to improve productivity in an electroplating wire, the technique is essential that a plurality of metal wires are passed in parallel into an electroplating solution and subjected to electroplating at the same time. It is very reasonable concept that a plurality of electrode plates erected vertically are set out in the plate thickness direction in an electroplating tank and a metal wire is passed though each gap between the electrode plates for this simultaneous electroplating. However, when it is brought into action, variations of electroplating amounts in a plurality of metal wires take place, it is very difficult to uniform the amounts. This trend becomes remarkable with increase in the number of metal wires to be electroplated at one time, which causes the productivity of the electroplating wire to be damaged.